A wireless network may employ orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA). In a cellular wireless network, each cell employs a base station (designated by Node B or eNB) that communicates with user equipment (UE), such as a cell phone, a laptop, or a PDA, which is actively located within its cell.
Initially, the base station transmits reference signals or pilot signals to the user equipment wherein the reference signals are based on a protocol shared by the base station and the user equipment. User equipment UE knows the signal, its frequency and its timing, so UE can generate a channel estimate based on the reference signal. Interference and noise impact the measured quality of the channel estimate.
In an OFDM or OFDMA system, different user equipments are scheduled on different portions of the system bandwidth. The system bandwidth is divided into frequency-domain groups or subbands that encompass resource blocks according to group size or subband size. A resource block is the smallest allocation unit available in terms of frequency granularity that is allocated to user equipment UE by a base station scheduler module. Each resource block RB consists of NRB contiguous OFDM/OFDMA sub-carriers. While the size of different resource blocks can in general vary, the same size is used across resource blocks for convenience so that the resource blocks size are as uniform as possible across the system bandwidth. A different user can potentially use or be allocated to use each of these resource blocks. In addition, a user can be scheduled on a portion of the system bandwidth having adjacent resource blocks inside. Non-adjacent allocation for each user is also possible.
The user equipment determines a channel quality indicator (CQI) for each of the resource blocks or for each of the subbands based on the channel estimation performed. The CQI metric is suitably a signal to interference noise ratio (SINR) after detection, a channel throughput measure, or other quality measure. The user equipment feeds back the CQI for each subband or even for each resource block to the base station. A higher CQI for a resource block allows a higher data rate transfer of information from the base station to the user equipment. The CQI for different subbands or for resource blocks can also be jointly encoded and compressed.
For systems with multiple transmit and multiple receive antennas, also called multi-input multi-output (MIMO) systems, improved throughput and/or robustness is suitably obtained by employing transmit pre-coding. To apply pre-coding on a MIMO system means that a certain transformation (typically linear or complex linear) for each RB is applied to the data stream(s) allocated to the RB prior to transmission via physical antennas. Scheduling involves base station allocation of UEs to RBs for determining the transformation. The number of independent data streams is termed the transmission rank. With pre-coding, the number of physical antennas does not have to be equal to the transmission rank. In this case, the precoding matrix is a P×R matrix, where P is the number of physical transmit antennas and R is the transmission rank (not more than P). Denoting the precoding matrix for each downlink RB as PM and the R independent data streams as an R-dimensional vector s, the transmitted signal via the P physical antennas is written as: x=PM s.
Precoding matrix PM for each RB in a given subband can be selected at the transmitter or receiver. For a frequency division duplex FDD system where the uplink and downlink channels are not reciprocal, precoding matrices to contribute to the matrix PM are more efficiently chosen at the receiver (user equipment UE) from a pre-determined set of matrices, termed the pre-coding codebook. Based on the channel/noise/interference estimate, which UE is in the best position to make, UE determines the precoding matrix selection based on the channel knowledge/estimation in each RB to optimize data throughput, for example. Therefore, the precoding matrix is also a function of the channel and its quality. The same codebook-based pre-coding scheme can also be used for TDD or half-duplex TDD/FDD.
Once this is done, the user equipment feeds back to the base station for each of its subbands or resource blocks, the precoding matrix and the CQI that will be achieved if that precoding matrix is used for the resource block in the transmission of data. For example, in the context of the 3GPP E-UTRA (Evolved Universal Terrestrial Radio Access) system deploying a 5-MHz transmission, 10 user equipments having feedback information pertaining to 25 resource blocks hitherto has apparently involved a high level of operational overhead information to be fed back to the base station to schedule them and form a precoding matrix PM for them.
In addition to the CQI and precoding matrix selection feedback, the user equipment can also select and feed back the transmission rank. While transmission rank selection may or may not be performed for each resource block, additional feedback overhead is involved.
Accordingly, further ways of reducing the amount of communications feedback between user equipment and base station are desirable.